The role of nuclear energy in environmental problems solving is substantiated. The energy sources with the lowest emission of CO2 are given accounting the emission throughout the whole life cycle of the energy source (hydroelectric, nuclear and wind power plants). The advantage nuclear power plants possess over these energy sources is substantiated. The data on the state of nuclear energy at present are given. The general problems are formulated of up-to-date nuclear energy using the overwhelming majority of thermal neutron reactors. The requirements to promising nuclear energy of the XXI century are summarized. The information is provided on the GIF - the Generation IV International Forum which was created in 2001. Two main ways of spent nuclear fuel (SNF) treatment are described. The single-phase cycle, the so-called open nuclear fuel cycle, implies the single use of nuclear fuel and the subsequent storage of it as waste products. The recycling of nuclear fuel is called closed fuel cycle and implies the return of unutilized uranium and accumulated plutonium into the cycle. The possibility of application of fast neutron reactors with sodium coolant as a part of the fourth generation nuclear energy systems in solving the problems of fuel supply and minimizing of the radioactive waste amount has been assessed. The data are given on the two-component structure of power system with thermal and fast neutron reactors combined by the closed fuel cycle which can provide the solution of general problems of the nuclear energy such as SNF accumulation and radioactive waste treatment, and supply the cardinal increase of the natural uranium use efficiency. The results of the analysis of the impact of nuclear power plants with BN-600 and BN-800 reactors on the environment and personnel are summarized. The possible variants of layout of fast neutron reactors with sodium coolant are presented. The advantage of pool type is substantiated from the point of view of safety increase. Given are the main characteristics of the BN-1200 reactor which ensure safety, low-waste operation, absence of limitations on raw material supplies for a long period of time. The design features of fast sodium reactors which ensure its high safety for personnel and environment are described.